Nate Benes

NET Nebraska provides streaming video from the floor of the Nebraska Legislature. I’d like to be able to listen to floor debate after the fact but the video from most sessions isn’t archived. Using some ffmpeg trickery, we can save a copy of the stream for later offline viewing.

NET makes two versions of the video available: one with “burned in” captions, and one without. They provide the video using HTTP Live Streaming over Amazon Cloudfront.

• Subtitles overlaid on video: https://d11v388ukuvwpo.cloudfront.net/livedirect/smil:leg-floor-ld-oc.smil/playlist.m3u8
• Subtitles embedded in the ATSC stream: http://d11v388ukuvwpo.cloudfront.net/livedirect/smil:leg-floor-ld.smil/playlist.m3u8

First, we save a copy of the live stream:

ffmpeg -i "https://d11v388ukuvwpo.cloudfront.net/livedirect/smil:leg-floor-ld.smil/playlist.m3u8" -bsf:a aac_adtstoasc -vcodec copy -c copy -crf 50 output.mp4

Then, we can extract the embedded ATSC subtitles to an SRT file:

ffmpeg -f lavfi -i "movie=output.mp4[out0+subcc]" -map s output.srt

Presto, easy offline viewing.